Method and apparatus for inspection of corrosion and other defects through insulation

ABSTRACT

Detection of corrosion and other defects in piping is needed to prevent catastrophic pipeline failure. Sensors, systems and methods are provided to enable detection of such defects. These apparatus and methods are configured to characterize pipe protected by insulation and conductive weather protection. The sensors may utilize inductive and/or solid state sensing element arrays operated in a magnetic field generated in part by a drive winding of the sensor. Multiple excitation frequencies are used to generate the magnetic field and record corresponding sensing element responses. Relatively high excitation frequencies may be used to estimate the properties of the weather protection and sensor lift-off while lower frequencies may be used to detect internal and external pipe damage. Linear arrays may be moved to generate damage images of the pipe providing size and location information for defects. Two dimensional sensor arrays may be used to provide imaging without moving the sensor.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/003,483, filed Jan. 21, 2016, now U.S. Pat. No. 9,823,179, issuedNov. 21, 2017, which is a continuation of U.S. application Ser. No.13/660,914, filed Oct. 25, 2012, now U.S. Pat. No. 9,255,875, issuedFeb. 9, 2016, which claims the benefit of U.S. provisional patentapplication, U.S. Ser. No. 61/600,355, filed Feb. 17, 2012 and U.S.provisional patent application, U.S. Ser. No. 61/551,232, filed Oct. 25,2011. The entire teachings of the above applications are incorporatedherein by reference.

GOVERNMENT SUPPORT

This invention was made with government support under DTPH56-10-T-00from Department of Transportation (DOT). The government has certainrights in the invention.

BACKGROUND

Pipes used for oil, gas, and chemical transportation or as part of arefinery or processing facility may be covered by coating that can beover four inches thick. Some coatings provide thermal insulation tominimize heat transfer between the surroundings and the hot or coldmaterials inside of the pipe. Other coatings are applied to decrease thebuoyancy of the pipe, such as a concrete weight coat for underwaterpipes, or to protect the pipe from mechanical damage.

To protect thermal insulation from mechanical damage, an aluminum orstainless steel weather protection (or sometimes called a “weatherjacket”) may be secured over the insulation and held in place by metalstraps along the length of the pipe.

FIG. 1A shows a cross-section of a pipeline 100 which has a pipe 101,insulation 103, and weather protection 105. The relative dimensions ofpipe 101, insulation 103 and weather protection 105 are illustrative.Actual pipes may have a wide range of parameters. Dimensions of noteinclude the pipe's outer diameter, OD, inner diameter, ID, and wallthickness, t_(s) (t_(s)=[OD−ID]/2). The insulation has a thickness,t_(i), and the weather protection has a thickness, t_(w).

Over time, pipe 101 can become corroded, reducing the integrity of thepipeline and increasing the risk of a catastrophic failure. Corrosioncan result in localized reduction in the wall thickness of the pipe,general wall loss over large areas, or localized pits. Material losscaused by corrosion can occur on both the inner and outer surfaces ofthe pipe as illustrated by the exemplary interior corrosion loss 107 andexterior corrosion loss 109. Other defect types that may be formed arecracks 106, 108 (e.g. stress corrosion cracking), welding anomalies,hard spots (local increases in material hardness), and dents.

If damage becomes severe, pipe 101 may fail, for example, as a result ofthe pressure of the transported gas/liquid. Inspections are needed toidentify locations of pipe that are likely to fail so that localreplacement or repair of the pipe section can be performed preemptivelyavoiding pipeline failure.

Inspection can be performed from the interior or exterior of the pipe.For interior inspection, a pipe inspection gauge (PIG) is inserted intothe pipe and wall thickness measurements are made using inspectiontechniques such as magnetic flux leakage (MFL) and ultrasound. As thePIG passes down the pipe, inspection data is recorded which can then beused to identify sections of the pipe requiring replacement or repair.

When the pipe diameter is small or the pipe was not constructed to allowfor the use of a PIG, interior inspection may not be practical. Theinspection may also be performed from the exterior. Conventionalexterior inspection techniques require that the weather protection andinsulation be removed from the pipe so that visual, ultrasonic, oranother inspection method can be performed. Such an inspection typicallytakes a considerable amount of time to perform. Further, the insulationand weather protection typically must be replaced after the inspectionadding to the expense of pipeline inspection.

SUMMARY

Detection of corrosion and other defects in piping is needed to preventcatastrophic pipeline failure. Sensors, systems and methods are providedto enable detection of such defects. These apparatus and methods areconfigured to characterize pipe protected by insulation and conductiveweather protection. The sensors may utilize inductive and/or solid statesensing element arrays operated in a magnetic field generated in part bya drive winding of the sensor. Multiple excitation frequencies are usedto generate the magnetic field and record corresponding sensing elementresponses. Relatively high excitation frequencies may be used toestimate the properties of the weather protection and sensor lift-offwhile lower frequencies may be used to detect internal and external pipedamage. Linear arrays may be moved to generate damage images of the pipeproviding size and location information for defects. Two dimensionalsensor arrays may be used to provide imaging without moving the sensor.

One aspect relates to a sensor for small-footprint scanning. The sensorhas at least one electrical connector, an array of sensing elements, andlead segments. The at least one electrical connector connectsinstrumentation to the sensor. The conducting drive loop has a primarysegment, a return segment, and a connecting segment. The primary segmentis a first distance from the array. The return segment is locatedoutside a surface defined by the array and primary segment and at asecond distance, at least three times the first, from the primarysegment. The connecting segment connects an end of the primary segmentto an end of the return segment. Finally the lead segments connect theends of the loop to the electrical connector.

In some embodiments, the array and primary segment are straight suchthat the surface is a planar surface.

In some embodiments, the array and primary segment are curved about acommon axis such that the surface is a cylindrical surface.

In some embodiments, the return segment is also curved about the commonaxis and is radially inward from the primary segment with respect to thecommon axis.

In some embodiments, the connecting segment is normal to the surfacedefined by the array and primary segment.

In some embodiments, the conducting drive loop comprises a plurality ofturns.

In some embodiments, the sensing elements of the array are inductiveloops.

In some embodiments, the sensing elements of the array are solid-statedevices.

Another aspect relates to a method of measuring material properties at atest location of a test object with such a sensor. The method comprisesflexing the sensor to a shape conforming to that of the test object;calibrating the sensor away from the test location on the test objectwhile maintaining the shape; and operating the sensor at the testlocation to measure the material properties.

Another aspect relates to a system for detecting damage in a hollowcylinder through an outer non-conducting layer which is furthersurrounded by a thin conducting layer. The system comprises a sensor, ananalyzer, and a processor. The sensor has an array of sensing elements,a drive winding, and a gap therebetween. The analyzer is operablyconnected to the sensor and configured to drive a current in the drivewinding at a plurality of measurement frequencies and to measure aresponse of each sensing element in the array at each measurementfrequency. The processor is configured to (i) estimate at least onematerial property of the thin conducting layer from at least theresponse of the sensing elements of the array at a first measurementfrequency among the plurality of measurement frequencies, (ii) estimatea spacing between the thin conducting layer and the hollow cylinder, anda wall thickness of the hollow cylinder from at least the response ofthe sensing elements of the array at a second measurement frequencyamong the plurality of measurement frequencies, where the secondmeasurement frequency is a lower frequency than the first measurementfrequency, and (iii) determine damage to the hollow cylinder based onthe estimates.

In some embodiments, the processor is further configured to estimate alift-off of the sensor from the conducting layer from at least theresponse of the sensing elements of the array at the first measurementfrequency.

In some embodiments, the processor is further configured to identifyregions of external and internal wall loss of the hollow cylinder usingthe estimated spacing between the thin conducting layer and the hollowcylinder and the estimated wall thickness of the hollow cylinder.

In some embodiments, the array of sensing elements is a first array ofsensing elements, the sensor further comprises a second array of sensingelements with a different gap to the drive winding, the impedanceanalyzer is further configured to measure responses of each sensingelement in the second array at a second plurality of measurementfrequencies that may or may not be different than the first plurality ofmeasurement frequencies, and the processor is configured to utilize theresponses from both the first and second arrays to determine damage.

In some embodiments, the analyzer is configured to drive the drivewinding at each of the plurality of measurement frequencies sequentiallyand measure the responses at each sensing element simultaneously using aparallel architecture.

In some embodiments, the processor is configured to use assumedproperties of the thin conducting layer if the estimated materialproperty of the thin conducting layer indicates that the thin conducinglayer has a conductivity-thickness product below a threshold, and to usethe estimated material property of the thin conducting layer if theestimated material property is above the threshold.

In some embodiments, the system further comprises a non-transientcomputer-readable storage medium operably connected to the processor,said storage medium having a precomputed database of sensor responses,and the processor is configured to generate the estimates by comparingthe sensing element responses to responses stored in the precomputeddatabase. The database stored in the non-transient computer-readablestorage medium may be responses precomputed based on a planarphysics-based layered media model, a numerical method, on aphysics-based model that accounts for the cylindrical shape, orgenerated in any other suitable way.

In some embodiments, the array of sensing elements is a first array andthe sensor further comprises a second array of sensing elements havingthe same gap to the drive winding as the first but located on anopposite side of the drive winding as compared to the first array. Theresponses of the first and second array may be used to correct for theeffects of motion and to measure the velocity of the sensor relative tothe hollow cylinder.

Yet another aspect relates to a method of operating such a system, andwhere the hollow cylinder is a pipe, the non-conducting layer is aninsulating layer surrounding the pipe, and the thin conducting layer isa metal weather protection. The method comprises (i) calibrating thesensor, (ii) positioning the sensor on the weather protection, and (iii)operating the analyzer and processor to obtain sensing element responsesat each of the plurality of measurement frequencies and to provide theestimates and determination of damage to the pipe.

In some embodiments, the processor outputs the determined damage to thepipe as an amount of internal corrosion loss and an amount of externalcorrosion loss at locations on the pipe.

In some embodiments the method further comprises (iv) moving the sensoralong the pipe while repeatedly operating the analyzer to obtain sensingelement responses; and (v) operating the processor to generate theestimates and provide an image of the pipe damage. When positioning thesensor, the array may be aligned in the circumferential direction of thepipe, and moving the sensor along the pipe comprises movement in theaxial direction of the pipe. As one alternative the array may be alignedin the axial direction of the pipe, and moving the sensor along the pipecomprises movement in the circumferential direction of the pipe. Thesensor may be moved at a constant velocity reducing the effect ofmagnetic convection on the sensing element responses. The velocity maybe recorded using a mechanical or optical encoder, or may be determinedusing the multi-frequency inspection data.

In some embodiments, calibrating the sensor comprises positioning thesensor in a non-conductive, non-permeable environment with the samecurvature as if the sensor is positioned on the metal weatherprotection.

Another aspect relates to a method for inspecting a complex pipingfeature. The method comprises (i) wrapping a pipe having insulation withweather protection, the weather protection having aconductivity-thickness product below a threshold; (ii) positioning amagnetoquasistatic sensor adjacent to the weather protection; (iii)measuring multi-frequency responses of the magnetoquasistatic sensor atthe complex piping feature; and (iv) estimating damage to the pipe bycomparing the multi-frequency responses to responses predicted assumingthe weather protection has a predetermined conductivity-thicknessproduct that is below the threshold.

In some embodiments the insulated pipe is wrapped with a stainless steelweather protection. The complex pipeline feature may be an elbow,T-joint, or protrusion portion of a pipeline.

In some embodiments the weather protection is a replacement weatherprotection, and the method further comprises identifying an existingweather protection as having a conductivity-thickness product in excessof the threshold; and removing the existing weather protection.

Yet another aspect relates to a method comprising (i) securing a sensorhaving a drive winding and a two-dimensional array of sensing elementsto an exterior surface of a test component, the test componentcomprising a hollow cylindrical conductor surrounded by an insulatinglayer; (ii) exciting the drive winding at a plurality of measurementfrequencies; (iii) measuring responses at each of the plurality ofmeasurement frequencies on each of the plurality of sensing elements inthe two-dimensional array; (iv) based on the measured responses,providing an estimate of wall loss for the hollow cylinder.

In some embodiments the sensor is secured to a bend region, T-joint,rounded protrusion or other complex feature of the test component. Insome embodiments, the test component is a pipe.

In some embodiments, the test component further comprises a thinconducting layer surrounding the insulating layer and the estimate takesinto account the properties of the insulating layer and the thinconducting layer.

In some embodiments, estimating at least one property of the insulatinglayer and the thin conducting layer is based on the measured responsesand the wall loss is estimated by accounting for the insulating layerand the thin conducting layer using such estimated properties. Though,in another embodiment the estimate of wall loss accounts for the thinconducting layer by assuming all relevant properties of the thinconducting layer.

The foregoing is a non-limiting summary of the invention, which isdefined by the attached claims.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1A shows a cross section of a pipeline;

FIG. 1B-C show illustrative complex pipeline features;

FIG. 2 shows a system with instrumentation and sensors for interrogatinga component under test according to some embodiments;

FIG. 3A-L shows configurations of the sensor for the system according tosome embodiments;

FIG. 4 is a flow diagram of a method for determining material loss andcontrolling performance of maintenance or repair actions on an insulatedpipe with weather protection;

FIG. 5A is a model of a sensor and insulated hollow cylinder withweather protection that may be simulated to predict the sensor response;

FIG. 5B is a simplified model of the sensor and insulated hollowcylinder with weather protection that may be simulated to predict thesensor response;

FIG. 6 is a flow diagram of a method for determining damage to aninsulated pipe having weather protection;

FIG. 7 is a flow diagram of a method for inspecting a complex pipingfeature such as pipeline elbow, T-joint, a rounded protrusion, bend, oranother complex feature; and

FIG. 8 is a flow diagram of a method for estimating wall loss of aninsulated hollow cylinder with weather protection.

DETAILED DESCRIPTION

The inventors have recognized and appreciated that there is a need forexterior pipe inspection that does not require the removal or subsequentreplacement of the pipe's insulation and weather protection (ifpresent). Methods and apparatus are provided that enable inspection ofthe pipe for corrosion under insulation (CUI) and other pipe defectsthrough the insulation and weather protection. Other defect types thatmay be detected include, for example and not limitation, stresscorrosion cracking (SCC), welding anomalies, hard spots (local increasesin material hardness), and dents.

The inventors have recognized and appreciated that deep penetratingmagnetoquasistatic (MQS) sensor arrays may be used to detect corrosionand other defects through insulation. In some embodiments, an array ofsolid-state sensing elements is used. Any type or combination of typesof MQS sensing elements may be used in the array. For example,magnetoresistive (MR), hall effect, or any other type of sensing elementcapable of responding to magnetic fields that have penetrated into orthrough the pipe wall may be used. Typically, sensing elements that aremarketed as MR are anisotropic magnetoresistive (AMR) elements. In someembodiments, giant magnetoresistive sensor (GMR) arrays are used.(Generally the term MR is used so as to include GMR as a special case.)MR elements directly detect magnetic fields as compared to inductiveelements which detect changes in magnetic fields. A discussion ofMR-Arrays and GMR-Arrays is provided in U.S. Pat. No. 6,992,482 (Jan.31, 2006) which is incorporated by reference in its entirety.

Attention is now turned to FIG. 2 which shows a block diagram of asystem 200 for inspecting the condition of a component under test 230.Component under test 230 may be a section of pipe such as that describedabove in connection with FIG. 1A. Though, component under test 230 maybe any suitable test article. System 200 includes an instrument 210 anda sensor 220. Instrument 210 is configured to provide excitation signals221 to sensor 220 and measure the resulting response signals 223 fromsensor 220. Measured response signals 223 may be processed to estimateproperties of interest, such as electrical properties (e.g.,conductivity, permeability, permittivity), geometric properties (e.g.,thickness, sensor lift-off), material condition, or any other suitableproperty or combination thereof. (Sensor lift-off is the effectivespacing between a sensing element and the component under test.)

Instrument 210 may include a processor 211, a user interface 213, memory215, an analyzer 217, and a network interface 219. Though, in someembodiments of instrument 210 may include other combinations ofcomponents. While instrument 210 is drawn as a single block, it shouldbe appreciated that instrument 210 may be physically realized as asingle “box”; multiple, operably-connected “boxes”, or in any othersuitable way. For example, in some embodiments it may be desired toprovide certain components of instrument 210 as proximal to sensor 220as practical, while other components of instrument 210 may be located atgreater distance from sensor 220.

Processor 211 may be configured to control instrument 210 and may beoperatively connected to memory 215. Processor 211 may be any suitableprocessing device such as for example and not limitation, a centralprocessing unit (CPU), digital signal processor (DSP), controller,addressable controller, general or special purpose microprocessor,microcontroller, addressable microprocessor, programmable processor,programmable controller, dedicated processor, dedicated controller, orany suitable processing device. In some embodiments, processor 211comprises one or more processors, for example, processor 211 may havemultiple cores and/or be comprised of multiple microchips.

Memory 215 may be integrated into processor 211 and/or may include“off-chip” memory that may be accessible to processor 211, for example,via a memory bus (not shown). Memory 215 may store software modules thatwhen executed by processor 211 perform desired functions. Memory 215 maybe any suitable type of non-transient computer-readable storage mediumsuch as, for example and not limitation, RAM, a nanotechnology-basedmemory, one or more floppy disks, compact disks, optical disks, volatileand non-volatile memory devices, magnetic tapes, flash memories, harddisk drive, circuit configurations in Field Programmable Gate Arrays(FPGA), or other semiconductor devices, or other tangible, non-transientcomputer storage medium.

Instrument 210 provides excitation signals for sensor 220 and measuresthe response signal from sensor 220 using analyzer 217. In someembodiments, analyzer 217 is an impedance analyzer. Though, analyzer 217may provide excitation and measure response signals in any suitable way.

Analyzer 217 may contain a signal generator 212 for providing theexcitation signal to sensor 220. Signal generator 212 may provide asuitable voltage and/or current waveform for driving sensor 220. Forexample, signal generator 212 may provide a sinusoidal signal at one ormore selected frequencies, a pulse, a ramp, wavelet, or any othersuitable waveform. Where interrogation of component under test 230 is tobe performed at multiple frequencies, the frequencies may be excitedsequentially, simultaneously, or in any suitable way.

Sense hardware 214 of analyzer 217 may include multiple sensing channelsfor processing multiple sensing element responses in parallel. Though,other configurations may be used. For example, sense hardware 214 mayinclude multiplexing hardware to facilitate serial processing of theresponse of multiple sensing elements. Sense hardware 214 may measure avoltage or current from on one or more sensing elements of sensor 220.In some embodiments, sensing hardware 214 represents the sensing elementresponse as a transimpedance. That is, the ratio of a voltage measuredacross the terminals of a sensing element to a drive current provided bysignal generator 212. Though, other measures may be used. Analysis ofdata sampling of portions of a waveform may be taken locally, forexample, with FPGAs or other hardware and/or software.

Sensor 220 may be any suitable sensing technology or combination ofsensing technologies. In some embodiments sensor 220 is a MQS sensorhaving a drive winding and an array of MQS sensing elements. The arraymay be arranged, for example, in a linear configuration or in atwo-dimensional configuration. Though any suitable configuration may beused.

A fixture 240 may be used to facilitate acquiring sensor data fromcomponent 230. For example, fixture 240 may be a mechanical structurethat facilitates positioning sensor 220 at desired locations relative tocomponent 230. In some embodiments, fixture 240 is a scanning fixtureeasing the movement of sensor 220 along a scan path on component 230.

Memory 215 of instrument 210 may store computer-executable softwaremodules that contain computer-executable instructions. These modules maybe read for execution by processor 211. Though, this is just anillustrative embodiment and other storage locations and execution meansare possible.

In some embodiments, the computer-executable software modules mayinclude a sensor data processing module, that when executed, estimatesproperties of the component under test. The sensor data processingmodule may utilize property grids stored in memory 115. The propertygrids are multi-dimensional pre-computed databases that relate single ormultiple frequency measurements obtained by analyzer 217 from sensor 220to material properties to be estimated. The sensor data processingmodule may take the property grids and sensor data and, using gridmethods, estimate material properties. (Grid methods are also discussedin U.S. Pat. No. 6,992,482.)

User interface 213 may include devices for interacting with a user.These devices may include, by way of example and not limitation, keypad,pointing device, camera, display, touchscreen, audio input and audiooutput.

Network interface 219 may be any suitable combination of hardware andsoftware configured to communicate over a network. For example, networkinterface 219 may be implemented as a network interface driver and anetwork interface card (NIC). The network interface driver may beconfigured to receive instructions from other components of instrument210 to perform operations with the NIC. The NIC provides a wired and/orwireless connection to the network. The NIC is configured to generateand receive signals for communication over network. In some embodiments,instrument 210 is distributed among a plurality of networked computingdevices. Each computing device may have a network interface forcommunicating with other the other computing devices forming instrument210.

The solid-state sensing elements may be used in combination with aninductive sensor array. For example, an inductive array and solid-statearray may be used in combination with one another as a single sensor 200as shown in FIG. 2. The drive winding 201 may be excited at multiplefrequencies to produce a drive current and magnetic field. The responseof the sensing elements will depend upon the magnetic field generatedproduced by the drive current and upon the proximity to the pipeline,the pipelines materials, and the condition the pipeline is in. Thesensing element responses may be measured and recorded and the processrepeated at multiple locations on the pipe. In some embodiments thearray is scanned along the pipe so that these repeated measurements maybe used to produce and image of the pipe, detecting and sizing defects.For example, axial or circumferential scans may be performed. Though,scanning may be performed in any suitable way. The measurements areprocessed to determine whether defects in the pipe are present so thatappropriate action may be taken.

FIG. 3A shows an example of sensor 300 comprising a drive winding 301,an array of sensing elements 303 and a connector 302. Sensor 300 may beused in the same ways as sensor 220 in system 200 of FIG. 2. Drivewinding 301 and array 303 are separated by a drive-sense gap 305.Drive-sense gap 305 is a characteristic distance between the nearestportion of drive winding 301 and array 303. Whether gap 305 is definedfrom center-to-center, edge-to-edge, or in another way is not critical.(In FIG. 3A gap 305 is shown as edge-to-edge, but this is merelyillustrative.) Drive-sense gap 305 may be selected to limit edge effectsfrom pipe ends and other geometry changes, straps securing weatherprotection, screws and other securing features, metal mesh in theinsulation, overlap regions of the weather protection, and othercomplicated geometries such as elbows and T-joints.

Connector 302 facilitates connection of the drive winding 301 andsensing elements to appropriate instrumentation such as instrument 210(FIG. 2). Connector 302 provides suitable electrical connections for thecomponents of sensor 300 and facilitates signal isolation between thevarious elements. In some embodiments, connector multiple physicalconnectors are used. Multiple physical connectors may be desired, forexample, to provide greater isolation of the drive signal from thesensing element returns.

In the illustrated embodiment, drive winding 301 is a “double-D” drivewinding. It has two large loops that may be driven to carry current inthe same direction in the adjacent winding sections of the loops. Theloops may have multiple turns to increase the amount of magnetic fluxcoupled to the array elements. A double-D drive winding constructed forexperimental validation had about 70 turns. Though, the loops may haveany suitable number of turns to produce a sufficient magnetic field to,for example, provide the necessary sensitivity to defects in the pipefor a given pipe geometry. In other embodiments drive winding 301 mayhave only a single drive loop or more than two loops. Any suitable driveconfiguration may be used as certain configurations may be moredesirable depending upon specific constraints of the application.

In some embodiments, analytical models that treat the drive winding as acurrent sheet are used to interpret sensing element measurements. Insuch embodiments, drive winding 301 may be wrapped with multiple turnsto simulate a planar current sheet. To achieve such an effect aflattened ribbon wire with a thin insulating coating may be wrapped withtight placement where each turn is adjacent to the next approximating anevenly distributed current sheet at low frequencies.

In some embodiments sensor 300 has multiple arrays of sense elementsplaced in a single drive loop. In this configuration each row has adifferent drive-sense gap, potentially increasing the independentinformation provided by the sense element when estimating materialproperties. FIG. 3B shows an embodiment of sensor 300 with a first array307 of solid-state sensing elements and a second array 309 of inductivesensing elements. The conducting loops of the inductive sensing elementsof array 309 may have one or more turns. The size of the loops anddistance of the loops from the drive winding may be chosen, for example,to provide suitable sensitivity to the properties of weather protection101 of pipeline 100 (FIG. 1A) while being insensitive to the thicknessof the insulation and properties of pipe 105 itself. As shown in FIG.3B, the solid-state elements of array 307 may be at a greaterdrive-sense gap than the inductive sensing elements of array 309.Generally, this will cause the solid-state elements to couple morestrongly with deeper penetrating magnetic field. In some embodiments thedistance is sufficient such that at suitable frequencies the magneticfield coupled to the solid-state elements is sensitive to the thicknessof pipe 105.

It may be desirable to provide sensor 300 such that the relativeposition of array 303 with respect to drive winding 301 may be readilyadjusted. In such a “variable-wavelength” sensor configuration, drivewinding 301 may be constructed separate from the sense elements. Thiscapability is particularly valuable during initial configuration of aninspection as the optimal distance from the sensing elements to drivewinding may not be known initially. Gap 305 may be varied to achievedifferent depths of sensitivity. (Increasing gap 305 will increase thedepth of sensitivity.) FIG. 3C shows a substrate 306 having array 309 ofinductive elements and array 307 of solid-state elements. Drive winding301 (FIG. 3B) may be formed, for example with a separate substrate andthe two substrates stacked and shifted relative to one another toachieve any desired drive-sense gap. In this example the solid-stateelements and the inductive elements are formed on the same substrate,though, in some embodiments, the arrays may be formed separately topermit independent adjustment of the distance to the drive windingloops.

It is noted that the choice of 9 elements is application specific and itshould be appreciated that the number and spacing of elements is notcritical. The number of elements in the array may be chosen in anysuitable way. For example, once array element size has been determinedbased on the sensitivity requirements, the elements may be configured tocompletely surround the pipe so as to permit scanning in the axialdirection of the pipe in one scan. In another embodiment, the arrays arewide enough so that the entire sensor spans between the metal strapssecuring the insulation and weather jacket. This configuration may beconvenient for scanning around the pipe and incrementally repositioningthe sensor along the pipe after each scan. Other configurations may alsobe used.

FIG. 3D shows another embodiment of sensor 300 where solid-stateelements may be arranged in array 307 and array 308. Arrays 307 and 308are positioned on both side of the drive. In some embodiments the twoarrays have the same drive-sense gap. Scanning in this configurationprovides redundancy and to facilitates for correction of errorsassociated with magnetic convection. Magnetic convection may becomerelevant when the magnetic diffusion time is sufficiently high (i.e.,the excitation frequency is sufficiently low) relative to materialtransport time (i.e., scan speed). Uncorrected for, convection willintroduce a phase shift that is not present when taking stationarymeasurements. Magnetic convection may be particularly significant whenmeasuring the thickness of thick steels a material commonly used for oiland gas pipe. The solid-state sense element locations may be adjusted toenhance or reduce sensitivity to convection. For example, if the arrayelements are centered within the loop of the double-D they will be lesssensitive to convection.

FIG. 3E shows yet another embodiment of sensor 300. Specifically, FIG.3E shows a detail of a two-dimensional array 310 and drive winding 311.As illustrated by the figure, array 310 has sensing elements in twodirections. Such a sensor may be used to image an area without movingthe sensor. Though array 310 is shown as inductive loops, it should beappreciated that any suitable magnetoquasistatic sensing element may beused.

FIG. 3F shows another embodiment of sensor 300. Here drive winding 320is a rectangular loop that is not coplanar with the sensing array 312.Drive winding 320 has a primary segment 321 nearest the array and whichis used to define the drive-sense gap 305. Primary segment 321 and array312 define a surface; in the coordinate system shown they specificallydefine the x-y plane. The drive winding further has a return segment 323outside the surface defined by primary segment 321 and array 312. Insome embodiments, return segment 323 is substantially parallel toprimary segment 321. A connecting segment 322 connects one end ofprimary segment 321 to an end of return segment 323. A fourth segment324 will connect the remaining end of either primary segment 321 orreturn segment 323 to lead segments 325. Drive winding 320 may havemultiple turns such that each of the four segments is formed frommultiple wires. Drive winding 320 may be formed with multiple turns inways similar to those described above.

In some embodiments, primary segment 321 and return segment 324 areseparated by at least three times the drive-sense gap 305. Suchseparation may be provided to reduce to the influence of the magneticfield produced by return segment 324 on the response of array 312.

Connecting segment 322 and fourth segment 324 are shown perpendicular tothe surface defined by primary segment 321 and array 312 (i.e., in thez-direction). This configuration is merely illustrative and segments 322and 324 may have different orientations.

FIG. 3G shows another embodiment of sensor 300. Here the sensor issimilar to that shown in FIG. 3F except the geometry is configured for acylindrically shaped test materials. The sensing elements of array 312follow a circular path with radius R₁ as does primary segment 321 ofdrive winding 320. Return segment 323 is shown following a radius R₂.Primary segment 321 and return segment 323 are joined radially byconnecting segment 322. While in the illustrated embodiment, R₂ is shownsmaller than R₁, this is merely illustrative, and the oppositeconfiguration may be used. Further, such segments may be flexible suchthat conformance to a component under test is made possible.

FIGS. 3H-K show drive winding 320 and array 312 from variousperspectives to provide clarity in the illustrated configuration. FIG.3L shows drive winding 320 and array 312 in cross-section andillustrates an embodiment where drive winding 320 has multiple turns.

In the embodiment of the inductive array shown in FIG. D, each elementof the inductive array has one turn and is etched on the same side of asingle layer of substrate. Though, it should be appreciated that theinductive elements may have multiple turns. Multiple turns may beachieved by forming one or more turns on each side of the substrate andusing via connections (a hole through the substrate that electricallyconnects traces on either side) to connect the turns on both sidesappropriately. The ends of the first and last loop may then be connectedto leads that connect the sensing loop to the instrumentation formeasurement. For example, two turns may be formed on one side of thesensor substrate and another two turns may be formed on the other sideof the sensor substrate and connected with vias. Multiple layers ofsubstrate can be stacked and connected with vias or with externalwiring. Elements can be formed with one or more turns of insulatedwires.

The sensor may be used to take multi-frequency sensor measurements.Specifically, at each frequency a response of the sensor may bemeasured. The measured response may be an impedance, admittance,voltage, current, or any other suitable parameter that may be used todetermine the properties of the pipe of interest. For the presentdiscussion the measured value is assumed to represent an impedance.Though, this is illustrative and chosen merely for convenience and anysuitable measure may be used.

The measurements may be related to the material properties of the pipeusing a suitable inverse method. In some embodiments, grid basedinversion methods such as those described in U.S. Pat. Nos. 6,992,482and 5,453,689 (and incorporated by reference in their entirety) areused. Though, any suitable inverse method may be used.

In some embodiments, the cross-section of the pipe is modeled using amultilayer model. In some instances a planar model may be sufficient,however, a cylindrical model will more accurately reflect the actualgeometry of the pipe. The model may be used, for example, to define asolution space for the inverse solution method used.

In an illustrative embodiment, a multilayer model is used to generatehyperlattices uses for the grid methods. A hyperlattice is amulti-dimensional database for relating a set of known values to a setof unknown values using the grid methods. (A grid is such a databasewhich relates 2 known parameters to 2 unknown parameters.) The model maybe configured to take treat certain parameters as constants while otherparameters are treated as variables to be estimated from the sensormeasurements.

Returning to FIG. 1A, the following parameters are defined for acylindrical model:

pipe outer diameter, O.D.,

pipe wall thickness, t_(s),

pipe conductivity, σ_(s),

pipe permeability, μ_(s),

insulator thickness, t_(i),

insulator conductivity, σ_(i),

insulator permeability, μ_(i),

weather jacket thickness, t_(j),

weather jacket conductivity, σ_(j), and

weather jacket permeability, μ_(j).

Additionally, because the sensor is assumed to be some small distancefrom the weather jacket, the sensor lift-off, h, must also be defined.Of course, alternative definitions of the parameters are possible (e.g.,pipe inner diameter, insulation O.D., weather jacket O.D.) and anysuitable set may be used. In some embodiments, certain parameters may betreated as frequency dependent. For example, the permeability of steelpipe may be dependent on frequency at the frequencies used forinspection. In some embodiments, parameters may have multiplecomponents. For example, permeability can be modeled as having anin-phase component and an out-of-phase component.

A planar model is shown in FIG. 5B. The parameters are the same exceptan O.D. is not included as by virtue of the choice of a planar model ithas essentially been assumed to be infinite.

The response of one or more sensors at one or more frequencies can beused to determine the value of these parameters. In practice, some ofthe parameters will be assigned assumed values and some will be treatedan unknown and estimated using the response of the sensors. The numberof unknowns that may be simultaneously determined will depend on thespecific sensor and instrumentation capabilities. For the presentillustration, a 6 unknown problem is posed and demonstrated inaccordance with the hardware available for testing. All other parametershave been assigned assumed values. Use of a greater or smaller number ofunknowns may be appropriate depending upon the nature of the materialbeing inspected and the instrumentation capabilities at hand.

A steel specimen having a 6⅝ inch O.D., a 0.28 inch wall thickness(nominal) 2.0 inch thick neoprene insulation (nominal), and 0.02 inchaluminum weather jacket (nominal) was studied. Based on the materials,certain parameters were given assumed values, reducing the number ofparameters that needed to be estimated from measurements. A planar modelwas used. The parameters were defined as follows:

pipe wall thickness, t_(s) UNKNOWN pipe conductivity, σ_(s) known pipepermeability, μ_(s), UNKNOWN - FREQUENCY DEPENDENT insulator thickness,t_(i) UNKNOWN insulator conductivity, σ_(i) 0 insulator permeability,μ_(i) μ_(o) weather jacket thickness, t_(j), UNKNOWN weather jacketconductivity, σ_(j) UNKNOWN weather jacket permeability, μ_(j) μ_(o)lift-off, h, UNKNOWN

The pipe conductivity was chosen as a constant based on the nominalsteel properties. The conductivity could be measured using, for example,a four point probe.

Thus the sensor measurements were used to estimate 6 unknowns: (1) thesensor liftoff, which represents the effective distance of the sensorfrom the weather jacket; (2) the thickness of the weather jacket; (3)the conductivity of the weather jacket; (4) the thickness of theinsulation; and the (5) wall thickness and (6) permeability of the steelpipe. Only the permeability of the steel pipe is treated as frequencydependent.

In order to solve for all unknowns, multi-frequency measurements aretaken on the inductive windings and the MR sensing elements. Typicalexcitation frequencies are between 30 Hz and 10 kHz. Though, anysuitable frequencies may be used for measurement.

The properties of the steel pipe and the weather jacket can be decoupledby using measurements from the inductive elements which may beconfigured so as to be insensitive to the properties of the steel pipe.This insensitivity may result from configuring the inductive arrayelements to have a low penetration depth. Accordingly, the sensorlift-off and the conductivity and thickness of the weather jacket may bedetermined independent of the insulation thickness and pipe properties.

Using relatively high frequencies where the magnetic field does notsignificantly penetrate through the steel pipe, the thickness of theinsulation is estimated from the MR sensors. The properties of theweather jacket and the liftoff are assumed in this estimation based onthe values obtained from the inductive elements.

At lower frequencies which penetrate through the steel pipe thethickness of the steel and the permeability of the steel may bedetermined. The properties of the weather jacket and insulation as wellas the liftoff are assumed based on the determination from the inductivesense element measurements and the high-frequency MR measurements. Thus,for each low frequency MR interrogation there are only two unknowns tobe solved for: the thickness and permeability of the steel. The accuracyof the permeability measurement may be biased by the choice ofconductivity for the steel which may not be entirely accurate for alllocations of the pipe. The permeability can be used to help qualify, forexample, the extent of damage or stress variations. The thickness of thesteel will reveal information about loss of wall thickness due tocorrosion. Also, the thickness of the insulation will appear to increasewhen the steel has corroded on the outside surface. Corrosion from theinside surface will result in reduced values of t_(s). Other defects,including but not limited to stress corrosion cracking (SCC), may beestimated as either changes in wall thickness and/or changes inpermeability.

Thus, by identifying unexpectedly large insulation thickness estimatesor unexpectedly small steel thickness estimates, locations of exteriorcorrosion and interior corrosion can be identified, respectively.

At locations where the pipe has had wall thickness reduction,appropriate maintenance action may be taken. For example, the insulationmay be removed and additional testing may be performed to verify thereduced wall thickness; the pipe may be repaired or replaced; or thelocation may be simply noted for future monitoring.

Method 400 shown in FIG. 4 is a method for determining material loss andcontrolling performance of maintenance or repair actions on an insulatedpipe with weather protection. Though, method 400 may be performed forany hollow cylindrical component, regardless of the component's intendedor actual use. Method 400 may be performed using system 200 show in FIG.2, though, any suitable instrumentation and sensors may be used.

At step 401, inductive sense element measurements are performed using asensor located on the exterior of the pipeline as show as system 500 inFIG. 5A. Specifically, FIG. 5A shows a pipeline 100 having a pipe 105,insulating layer 103, and a weather protection 101. The inductive senseelement measurements may be performed at one or more frequencies suchthat the lift-off and weather protection properties may be estimated asnecessary. Such measurements may be performed in ways described hereinor in any suitable way.

At step 403, the lift-off of the sensor (h in FIG. 5A), the thickness ofthe weather protection (t_(j)), and/or the conductivity of the weatherprotection (σ_(j)) are estimated using the inductive sense elementmeasurements. The measurements may be used to estimate these propertiesin any suitable way. For example, the response of the sensor may bepre-computed using a physics-based model and the responses stored in adatabase. The physics based model may account for the cylindrical shapeof the pipeline or may approximate the pipeline as planar. In someembodiments the system 500 is modeled using a numerical method such asthe finite element method (FEM) or any another suitable numericalmethod.

The measured inductive sense element responses may be related topre-computed responses stored in the database using grid methods. Suchgrid methods may then provide estimates of the desired parameters. Othermethods may be used to estimate the parameters. For example, anysuitable form of an iterative estimation approach may be used.

In some embodiments, one or more of the lift-off, weather protectionconductivity, and weather protection thickness may be assumed. Forexample, the thickness of the weather protection layer may be assumedand only the lift-off of the sensor and the conductivity of the weatherprotection layer estimated at step 403. Though this example is merelyillustrative and any suitable combination of these parameters may beestimated at step 403.

At step 405, solid-state sensor measurements are taken. Thesemeasurements are taken at relatively high frequencies compared to thesolid-state measurements taken at step 409 (described below). Thefrequencies may be selected such that the sensor is insensitive to thetotal pipe wall thickness. This condition enables the pipe wallthickness to be approximated as infinite simplifying analysismomentarily. In some embodiments the frequencies used at step 405 andstep 409 will be lower than the excitation frequencies used at step 401.

At step 407, using the prior estimated or assumed lift-off and weatherprotection conductivity and thickness as well as the measurements fromthe solid-state sensing elements, the thickness of the insulating layeris estimated. The permeability and conductivity of the pipe may also beestimated. For example, in some embodiments the conductivity of the pipeis assumed (e.g., based on prior measurements) and the permeability ofthe pipe is also estimated. Because the permeability and conductivity ofthe pipe affect the sensor response in similar ways any errors in theassumed conductivity of the pipe may be accounted for in thepermeability estimate while still achieving a accurate estimate of theinsulation thickness. Step 407 assumes that the conductive senseelements and solid-state sense elements are integrated into the samesensor such that the lift-off in both configurations will besubstantially identical. If separate sensors are used it may benecessary to estimate the lift-off for both configurations. Theestimation may be performed in ways similar to those described inconnection with step 403 or in any suitable way. FIG. 400 shows apotential model that may be used of the piping system at step 407.

Note that the insulation thickness as used here is not necessarily thetrue insulation thickness. Rather it would be more accurate to refer tothe thickness being estimated as the distance between the outer surfaceof the pipe and the inner surface of the weather protection layer.Generally these distances should be substantially identical, however,material loss to the exterior of the pipe will effectively increase theestimated value. Because the thickness of the insulation may be wellknown this difference may be attributed to material loss in the pipe asdescribed later in connection with steps 413-415.

At step 409, relatively low frequency measurements are taken using thesolid-state sensing elements of the sensor. The frequencies may beselected such that the sensor response is dependent upon the wallthickness of the pipe. It is assumed here that the same solid-statesensor array is used at both steps 405 and 409. In some embodiments asensor may be configured to use two different arrays of solid stateelements with different drive-sense gaps such that the field penetrationnecessary to measure the pipe wall thickness can be achieved withoutnecessarily using lower frequencies than those used at step 405.

At step 411, the permeability and thickness of the pipe are estimatedusing (i) the lift-off and the weather protection conductivity andthickness estimated/selected at step 403; (ii) the insulation thicknessestimated at step 407; and (iii) the sense element measurementsperformed with the solid-state sensors at step 409. Note that thepermeability estimate made at step 407 of the pipe is not assumed here.Rather, it is re-estimated with the thickness of the pipe.

At step 413, a determination is made as to whether material loss in thepipe has occurred. Material loss may occur either internally orexternally to the pipe. In some embodiments, internal material loss isdifferentiated from external material loss. Whether interior or exteriorwall loss has occurred in the pipe may be based on the measurements ofwall thickness and insulation thickness. The nominal wall thickness andnominal insulation thickness may be used with these measurements toidentify whether the type of wall loss (if any) that has occurred. Forexample, one algorithm for determining whether exterior wall loss hasoccurred is to simply compare the nominal insulation thickness with themeasured insulation thickness and conclude wall loss has occurred if thelatter is greater than the former. Another algorithm may furtherdetermine there is exterior wall loss if the measured insulationthickness is larger than the nominal insulation thickness and the wallthickness is less than the nominal wall thickness of the pipe by atleast the same amount. Though, the identification of wall loss in thepipe may be performed in any suitable way. The results of thedetermination at step 413 may be output to a human readable outputdevice or recorded or transmitted in any suitable way.

If it is determined that material loss has occurred at step 413,maintenance or repair actions may be performed. For example, the pipemay be replaced or patched at the location where the material loss hasoccurred. After repair or maintenance of the pipe at step 415, or thedetermination at step 413 that material loss has not occurred, method400 terminates.

It should be appreciated that the steps of method 400 and the order ofthese steps are merely illustrative. The order of steps may be modifiedand based on the needs of the specific application and capabilities ofthe instrumentation and sensing apparatus. For example, all measurementsteps (401, 405, 409) may be performed before any of the estimationsteps are performed (403, 406, and 411). As another example, in someembodiments, all measurement results are processed simultaneously toprovide simultaneous estimates of the unknown parameters.

Returning briefly to system 500 shown in FIG. 5A. System 500 includes asensor 220 and a pipeline 100. System 500 may be used as a model of thesensor pipeline configuration for predicting the responses of sensor220. In system 500, The sensor 220 is modeled as being separated fromthe pipeline (specifically the weather protection) by a lift-off, h. Thepipe may be modeled as having an O.D., a wall thickness, t_(s), a knownconductivity and an unknown permeability. The conductivity may beestimated experimentally or using literature values. The insulatinglayer surrounding the pipe may be modeled as having a thickness t_(i), apermeability of free space, and zero conductivity. The weatherprotection layer may be modeled as a conductive layer having a thicknesst_(j) and the permeability of free space. In embodiments where theconductivity-thickness product of the weather protection is sufficientlylow (e.g., below a set threshold) the weather protection may be ignoredor modeled using assumed values. The weather protection may be ignoredin typical configurations where stainless steel is used as the weatherprotection. (Stainless steel has a substantially lower conductivity thanaluminum, a commonly used weather protection material.)

While sensor 220 is shown wrapping around approximately one quarter ofthe circumference of the pipeline, this is merely illustrative. In someembodiments, sensor 220 may wrap completely around pipeline 100 orenable complete imaging of the pipe circumference in 2, 3, 4 or moreplacements.

FIG. 5B shows system 510, an alternative representation of thesensor/pipeline system using plainer coordinates. Here the outerdiameter of the pipe is assumed to essentially be infinity. All otherparameters are substantially analogous to those shown in FIG. 5A.

FIG. 6 shows method 600 for determining damage to an insulated pipehaving weather protection. Method 600 may be performed in the system 200shown in FIG. 2 where component under test 230 is the pipeline. Though,any suitable system may be used.

At step 601, the sensor is calibrated. The sensor may be calibrated inair or using a reference material. In some embodiments, the sensor isheld with the same geometric shape it will assume when performinginterrogation of the pipeline (i.e., having the same shape taken at step603). Air may be simulated using non-conducting, non-permeable materialssuch as plastics, ceramics or other such materials that do not affectmagnetoquasistatic fields.

At step 603, the sensor is positioned on the pipeline's exterior. Thepipeline is assumed to have a pipe surrounded by a non-conductinginsulating layer which is further surrounded by a thin conducting layer.

At step 605, an analyzer is operated to drive the sensor and interrogatethe sensing elements to determine responses from each element atmultiple frequencies. In some embodiments the analyzer is an impedanceanalyzer. Though the sensor response may be measured in any suitableway.

At step 607, a processor is operated to estimate parameters of the pipebased on the multi-frequency measurements of the sensor performed atstep 605. Based on these estimated parameters, it is determined at step607 where damage has occurred to the pipeline. In some embodiments, theprocessor outputs the determined damage to the pipe as an amount ofinternal corrosion loss and an amount of external corrosion loss atlocations on the pipe.

In some embodiments of method 600 the sensor is moved along the pipewhile repeatedly operating the analyzer to obtain sensing elementresponses. The movement may be performed as a scan or by tiling. Themeasured responses may be processed to generate the estimates andprovide an image of the pipe damage.

When tiling (also called “leap-frogging”) a two dimensional array may beused to accelerate the rate of imaging of the pipe using a tilingprocedure.

When scanning, the sensor may be placed on the pipe such that the arrayis aligned in the circumferential direction of the pipe and moving thesensor along the pipe comprises movement in the axial direction of thepipe. In another scanning embodiment, the sensor is placed on the pipesuch that the array is aligned in the axial direction of the pipe andmoving the sensor along the pipe comprises movement in thecircumferential direction of the pipe.

When scanning the sensor during measurement magnetic convection mayaffect the response of the sensor. Various techniques may be used toaccount for the convection effects. The sensor may be moved at aconstant velocity, reducing the effect of magnetic convection on thesensing element responses. The velocity information of the sensor may berecorded during the movement and the estimates provided at step 605 mayutilize the velocity information to correct for magnetic convection.

Method 700 shown in FIG. 7 is a method for inspecting a complex pipingfeature such as pipeline elbow 110 shown in FIG. 1B, T joint 120 shownin FIG. 1C, a rounded protrusion, bend, or another complex feature.

At step 701 weather protection having a conductivity-thickness productbelow a threshold is wrapped about a pipe having insulation. Theconductivity-thickness product threshold may be selected such that thesensor configuration to be used for inspecting the pipe through theinsulation and weather protection can image the pipe with sufficientaccuracy when the weather protection is ignored or its properties areassumed without measurement. In some embodiments the insulated pipe iswrapped with stainless steel with a typical weather protection thicknesssuch as 0.020 in. (0.5 mm). In some embodiments the weather protectionis a replacement weather protection that replaces an existing weatherprotection that has a conductivity-thickness product that is above thethreshold. For example, stainless steel weather protection may replacealuminum weather protection of nominally the same thickness.

At step 703 a magnetoquasistatic sensor is placed adjacent to theweather protection. The sensor may be a two-dimensional solid statearray such as that described in connection with FIG. 3E, though, anysuitable sensor may be used.

At step 705 multi-frequency responses of the magnetoquasistatic sensorare measured at the complex piping feature.

At step 707 damage to the pipe is estimated. Pipe damage may be estimateby comparing the multi-frequency responses to responses predictedassuming the weather protection has a predeterminedconductivity-thickness product that is below the threshold. For example,the conductivity-thickness product may be assumed to be zero (i.e., theweather jacket is completely ignored). In some embodiments the damage tothe pipe is output as an amount of internal corrosion loss and an amountof external corrosion loss at locations on the pipe. The output may beprovided in any suitable format, such as on a display or on another formof human readable output. Method 700 ends after step 707.

For simplicity in the discussion above the permeability of the weatherprotection was assumed to be that of free space. If the weatherprotection is permeable (i.e., has a relative permeability greaterthan 1) the conductivity-permeability-thickness product may be used asthe relevant metric.

Attention is now turned to method 800 show in FIG. 8.

At step 801, a sensor having a drive winding and a two-dimensional arrayof sensing elements is secured to an exterior surface of a testcomponent, the test component comprising a hollow cylindrical conductorsurrounded by an insulating layer. In some embodiments the testcomponent is a complex feature such as an elbow, T-joint, a roundedprotrusion, bend, or another complex feature. In some embodiments thetest component is a complex feature of a pipeline.

At step 803, the drive winding of the sensor is excited at a pluralityof measurement frequencies.

At step 805, responses at each of the plurality of measurementfrequencies are measured on each of the plurality of sensing elements inthe two-dimensional array.

At step 807, based on the measured responses, estimates of wall loss forthe hollow cylinder are provided.

In some embodiments of method 800 the test component further comprises athin conducting layer surrounding the insulating layer and the estimatetakes into account the properties of the insulating layer and the thinconducting layer. At least one property of the insulating layer and thethin conducting layer may be estimated based on the measured responses.Further the estimate of wall loss may account for the insulating layerand the thin conducting layer using such estimated properties.

MR sense element circuitry is now discussed to illustrate variousconfigurations where MR elements are used in the sensor array. MR senseelements have many features that can be taken advantage of to providemore accurate field measurements. Selecting the MR sense element chipthat has the most desirable characteristics given the application isnecessary. By operating the MR element at a low-power, thermal drift canbe minimized. By operating the MR element in a low magnitude field,nonlinearities intrinsic to MR elements can be minimized. MR chips haveinternal set-reset straps which allow for repolarization of thepermalloy. Using the straps after a certain number of measurementsallows for maximization of sensitivity and linearity. The internaloffset straps may be used to (i) provide dynamic air recalibration tocompensate for all types of drift including thermal drift; (ii) offsetthe Earth's magnetic field to increase MR elements dynamic range andreduce sensitivity to orientation changes; and (iii) in higher powerapplications, to operate as bucking coils which cancel the nominal fieldpresent during an air measurement or a measurement on a representativematerial, increasing the MR element's dynamic range.

Using the internal offset straps can cause problems at higher fieldstrengths due to the thermal effects on the sensor die. In this case anexternal winding can be used as well to achieve the same effects aslisted above.

Since the MR element directly detects magnetic fields and not changes inmagnetic fields like inductive elements, bias fields can also be usedadvantageously. Bias fields can be used to relocate the MR element'soperational point on the B-H curve with a number of effects. Forexample, bias fields can be varied to detect changes in impedance due todispersive properties of a material. Also, high power bias fields can beused to saturate a material, therefore raising the frequency necessaryto achieve sensitivity through the material.

Attention is now turned to various details related to imaging. With alarge 2-D array, and even with a 1-D array, it is possible to build animage with stationary measurements, leapfrogging from location tolocation. At very low frequencies avoiding movement of the sensor avoidsthe convective effects. When the magnetic diffusion time is sufficientlyhigh (the excitation frequency is sufficiently low) relative to materialtransport time (scan speed), there is a phase error associated withmoving while taking a measurement. This is especially significant whenmeasuring the thickness of thick steels. There are multiple approachesto dealing with this effect: (i) as mentioned earlier, building an imageusing stationary measurements and leapfrogging avoids this effect; (ii)by modeling the effect and then monitoring the speed using an encoder itcan be accounted for; (iii) instead of using an encoder, multiplefrequencies and multiple sensor rows with different drive-sense gaps canbe used to estimate the sensor's speed in real-time; (iv) placing an MRelement intelligently within the single-D or multiple-D drive the effectcan be systematically reduced; (v) by scanning at a constant speed thephase error can be accounted for as a constant offset instead of atime-varying error.

Finally, the convective effect can be used advantageously. By usingmultiple frequencies and multiple sensors and/or multiple drives withdifferent drive-sense gaps, the speed of the sensor can be estimated inreal-time.

A discussion is now made of using high-permeability materials, such asmu-metals or ferrites, behind the sensor to force a higher percentage ofthe field created by the drive winding into the material under test.This can increase the sensor response due to material changesdrastically. In addition to increasing signal levels, themu-metal/ferrite acts a shield to external noise. Furthermoremu-metal/ferrite can be used as a backing in a non-planar form (forexample curving around the edges of the sensor) in order to shape thefield. This can be used to reduce sensitivity to unwanted geometriessuch as plate or pipe edge effects.

In order to be used effectively, the mu-metal/ferrite must be accuratelymodeled. The properties of the mu-metal/ferrite are most effectivelydetermined by using the sensor for which it will serve as the backing.The sensor may be calibrated in air without the mu-metal/ferritebacking. Then, with this calibration, a multi-frequency measurement ismade after the mu-metal/ferrite has been mounted. This measurement isused to determine the properties of the mu-metal/ferrite that should beassumed in any future measurements. Since mu-metals properties inparticular are sensitive to deformation, it is best to make thismeasurement in the form the sensor will be used.

Many of the applications require many material properties to beestimated simultaneously. This high dimensional property inversion isdifficult and often requires a combination of many different approaches.

An n-unknown multiple frequency inversion without any hierarchicaltreatment or segmented field information. For example, on a pipe with noweather jacket, the unknowns may be selected as liftoff, pipe thickness,permeability, and conductivity. In some applications all four unknownscan be estimated independently. In others, one or of more be consideredconstant. In a configuration with a weather jacket, this can add theweather jacket conductivity, permeability, thickness and distance fromthe weather jacket to the pipe as unknowns.

A hierarchical approach can be used to increase numerical stability andaccuracy of the multiple unknown inversion. Property effects can besystematically separated from one another by using specific frequenciesto estimate the properties they are most sensitive to.

A segmented field approach with multiple sense elements and/or multipledrives may help increase measurement independence between near and farsurface properties.

When using one segment of the sensor to provide information to othersegments of the sensor, it may be necessary to use a numerical method toconvert material signatures from one segment to the other due todiffering sensor footprints. A specific application of this is the needto use near segments to correct far segments for weather jacketvariations such as straps and overlap regions.

A sensor with both inductive and MR elements may allow for an individualsensor to operate effectively over a larger frequency range, takingadvantage of the inductive sensor's sensitivity to the rate of change ofthe magnetic field at higher frequencies and the MR sensor's sensitivityto the magnetic field at lower frequencies.

Repeating scan at multiple lift-offs may increase measurementindependence between various material properties. Similarly, a singlescan can simulate multiple liftoffs by placing multiple drives atdifferent heights relative to the sense element and alternatingexcitation between the two drives.

A 4-point-probe may be used separately to measure conductivityindependent of other material properties

Steel may need to be modeled as having a complex and/or frequencydependent and/or depth dependent permeability.

Analytical and numerical methods or some combination of the two can beused to correct for and accurately size local property variations. Thisincludes but is not limited to weather jacket straps and overlap regionsas well as pipeline localized defects.

Calibration can be performed in a number of manners including but notlimited to air calibration, single or multiple point referencecalibration, air and reference part calibration, air and shuntcalibration, any number of more complicated calibration procedures.

The use of the cylindrical extension of the models provides theopportunity for a more complicated air calibration procedure. Modifyingthe radius of the sensor (or any other systematic change to a sensorthat changes the sensors response in air) provides an independent aircalibration point allowing for the estimation of the parasitics of theimpedance instrument system without the use of a shunt. This cansimplify the measurement procedure.

Complex Geometry Locations

In locations with more complicated geometries it may become moreimportant to use techniques such as 2-D stationary arrays to reduceconvective effects and take measurements at known distances away fromthe complicating geometry. Analytical and numerical models may becombined to more carefully account for the complicating geometries. Thepipeline may be adapted with inspection ease in mind. Reducing thedifficulty of the inspection process can be achieved by using lowerconductivity-permeability-thickness product weather protection materialas compared to aluminum or magnetic steel materials. Some suggestedmaterials include but are not limited to stainless steel, plastic,polymer, fiber glass, carbon fiber composite, Kevlar, materials used inflexible armor.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Accordingly, the foregoing description anddrawings are by way of example only.

The above-described embodiments of the present invention can beimplemented in any of numerous ways. For example, the embodiments may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer. Additionally, acomputer may be embedded in a device not generally regarded as acomputer but with suitable processing capabilities, including a PersonalDigital Assistant (PDA), a smart phone or any other suitable portable orfixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audible format.

Such computers may be interconnected by one or more networks in anysuitable form, including as a local area network or a wide area network,such as an enterprise network or the Internet. Such networks may bebased on any suitable technology and may operate according to anysuitable protocol and may include wireless networks, wired networks orfiber optic networks.

Also, the various methods or processes outlined herein may be coded assoftware that is executable on one or more processors that employ anyone of a variety of operating systems or platforms. Additionally, suchsoftware may be written using any of a number of suitable programminglanguages and/or programming or scripting tools, and also may becompiled as executable machine language code or intermediate code thatis executed on a framework or virtual machine.

In this respect, the invention may be embodied as a computer readablemedium (or multiple computer readable media) (e.g., a computer memory,one or more floppy discs, compact discs, optical discs, magnetic tapes,flash memories, circuit configurations in Field Programmable Gate Arraysor other semiconductor devices, or other tangible computer storagemedium) encoded with one or more programs that, when executed on one ormore computers or other processors, perform methods that implement thevarious embodiments of the invention discussed above. The computerreadable medium or media can be transportable, such that the program orprograms stored thereon can be loaded onto one or more differentcomputers or other processors to implement various aspects of thepresent invention as discussed above.

In this respect, it should be appreciated that one implementation of theabove-described embodiments comprises at least one computer-readablemedium encoded with a computer program (e.g., a plurality ofinstructions), which, when executed on a processor, performs some or allof the above-discussed functions of these embodiments. As used herein,the term “computer-readable medium” encompasses only a computer-readablemedium that can be considered to be a machine or a manufacture (i.e.,article of manufacture). A computer-readable medium may be, for example,a tangible medium on which computer-readable information may be encodedor stored, a storage medium on which computer-readable information maybe encoded or stored, and/or a non-transitory medium on whichcomputer-readable information may be encoded or stored. Othernon-exhaustive examples of computer-readable media include a computermemory (e.g., a ROM, a RAM, a flash memory, or other type of computermemory), a magnetic disc or tape, an optical disc, and/or other types ofcomputer-readable media that can be considered to be a machine or amanufacture.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects of the present invention asdiscussed above. Additionally, it should be appreciated that accordingto one aspect of this embodiment, one or more computer programs thatwhen executed perform methods of the present invention need not resideon a single computer or processor, but may be distributed in a modularfashion amongst a number of different computers or processors toimplement various aspects of the present invention.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconveys relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. A method comprising acts of: securing a sensorhaving a drive winding and a two-dimensional array of sensing elementsto an exterior surface of a test component, the test componentcomprising a hollow cylindrical conductor surrounded by an insulatinglayer; exciting the drive winding at a plurality of measurementfrequencies; measuring responses at each of the plurality of measurementfrequencies on each of the plurality of sensing elements in thetwo-dimensional array; based on the measured responses, providing anestimate of condition for the hollow cylinder conductor.
 2. The methodof claim 1 wherein the condition is related to stress in the hollowcylinder conductor.
 3. The method of claim 1 wherein the condition isrelated to damage.
 4. The method of claim 1, wherein the condition isrelated to corrosion damage.
 5. The method of claim 1, wherein thecondition is related to cracking damage.
 6. The method of claim 1,wherein the sensor is stationary during the act of exciting the drivewinding.
 7. The method of claim 1, further comprising moving the sensoralong the hollow cylindrical conductor and, while moving, continuing theact of exciting the drive winding and repeating the act of measuringresponses.
 8. An apparatus for measuring the condition of a hollowcylindrical material system, the system comprising: a sensor having anarray of sensing elements and a drive conductor, the sensor configuredto be positioned outside the hollow cylindrical material system; ananalyzer configured to measure a first response of the sensor on each ofthe sensing elements at a first measurement frequency, and measure asecond response of the sensor on each of the sensing elements at asecond measurement frequency that is lower than the first measurementfrequency; and a processor configured to estimate a spacing between thesensor and a nearest conducting surface of the hollow cylindricalmaterial system from at least the first response, and estimate acondition of the hollow cylindrical material system from at least thesecond response.
 9. The apparatus of claim 8, wherein the condition theprocessor is configured to estimate is related to stress.
 10. Theapparatus of claim 8, wherein the condition the processor is configuredto estimate is related to damage.
 11. The apparatus of claim 10, whereinthe hollow cylindrical material system comprises a pipe and theprocessor is further configured to differentiate damage at an innerradius of the pipe from at an outer radius of the pipe.
 12. Theapparatus of claim 8, wherein the condition the processor is configuredto estimate is related to crack damage.
 13. The apparatus of claim 12,further comprising a display configured to show a location of crackdamage relative to the sensing elements of the array.
 14. The apparatusof claim 8, wherein the condition the processor is configured toestimate is related to corrosion damage.
 15. The apparatus of claim 8,wherein the analyzer is configured to excite the drive conductor of thesensor with the first and second measurement frequencies simultaneously.16. The apparatus of claim 8, wherein the analyzer is configured tomeasure the first response on at least two of the sensing elements ofthe array simultaneously.
 17. The apparatus of claim 8, wherein thedrive winding has at least one rectangular shaped loop with longer sidesand shorter sides, and the array of sensing elements has at least onerow parallel with the longer sides of the drive winding.
 18. A systemfor measuring properties of a layered material, the system comprising: asensor with a drive winding and a plurality of sensing elements, thesensor configured to be placed proximal to the proximal layer of thelayered material; an instrument connected to the drive winding and theplurality of sensing elements, the instrument configured to generate acurrent in the drive winding at a plurality of frequencies, and tomeasure at each frequency a response of the sensor for each sensingelement, wherein the response at each frequency comprises two componentsand the plurality of frequencies comprise a high frequency, a mediumfrequency, and a low frequency; and a processor configured to estimate afirst property of the proximal layer based at least in part on theresponse at the high frequency, estimate a second property of anintermediate layered based at least in part on the response at themedium frequency, and estimate a third property of a deepest layer ofinterest based at least in part on the response at the low frequency.19. The system of 18, wherein the two components of the response are areal component and an imaginary component.
 20. The system of claim 19,wherein the plurality of sensing elements are magnetoresistive sensingelements; and the response are the real and imaginary components of afunction that combines an output of the magnetoresistive sensing elementand an estimate of the current in the drive.